During the last years, the production and studies concerning non-fiction cinema have experienced a great increase. However, there is no consensus about its definition and about the key features of this genre. This paper develops the proposal of Carl R: Plantinga on those issues, as it is explained mainly in his work "Rhetoric and Representation in Non-Fiction Film." Following this starting point, the article investigates the philosophical roots of his work, and studies in depth the application of Charles S. Peirce's triadic classification of the sign. Finally, the paper tries to point out that the efficacy of the indexical condition of the non-fiction cinema as a key feature to define this genre is valid if it is linked to the two other sign's dimension, the iconic and the symbolic.